1. Field of the Invention
The present invention relates to overhead camshaft engines, particularly double overhead camshaft (DOHC) engines wherein two camshafts are disposed over the cylinder head of an engine.
2. Description of the Prior Art
A valve operating device including two camshafts disposed over the cylinder head of an engine is well known in the art as is disclosed in the Japanese Provisional Patent Publication No. 57-2409 and also shown in FIGS. 8 and 9.
Referring to FIGS. 8 and 9, indicated by the reference numeral 1 is a cylinder head, and over the cylinder head 1 there are disposed two camshafts 2, 3 which are parallel to each other and rotatably supported by bearings 4, 5. The camshafts 2, 3 are driven by a timing belt (not shown) or the like in such a manner as to be rotatable in timed relation to revolution of an engine. The camshafts 2, 3 are respectively formed with cams 8, 9 for actuation of intake valves 6 and exhaust valves 7. In the example shown, each cylinder of the engine is provided with two intake valves 6 and two exhaust valves 7. Due to this, the camshafts 2, 3 are respecively provided with two cams 8, 9 in such a manner as to interpose therebetween the bearings 4, 5.
Indicated by the reference numeral 10 are valve lifters for the intake and exhaust valves 6, 7 and by 11 are valve springs for same. When the camshafts 2, 3 rotate, the valve lifters 10 are pushed by the cams 8, 9 against the springs 11, causing the intake and exhaust valves 6, 7 to open at a predetermined timing.
In the above described valve operating device, it is desirable to arrange the two camshafts 2, 3 as close as possible to each other for thereby making the included angle .theta. between the axes of the intake and exhaust valves 6, 7 as small as possible. Reduction in the included angle .theta. contributes to reduction in the surface area of a combustion chamber 12 since the intake and exhaust valves 6, 7 constitute in part the wall of the combustion chamber 12, whereby to make it possible to improve the combustion efficiency. A large included angle .theta. results in a large inclination of the ceiling wall of the combustion chamber 12 and an increased surface area of same and a reduced combustion efficiency.
When, however, to arrange the two camshafts 2, 3 closer to attain a small includedc angle .theta., there arises a problem of the interference between the fastening members for fastening the cylinder head 1 to the cylinder block 13 and the camshafts 2, 3, i.e., between the head bolts 14 and the camshafts 2, 3. For this reason, it has been impossible to reduce the distance between the two camshafts 2, 3 to a satisfactory extent. The minimum distance between the two camshafts 2, 3 is the sum of the distance between the axes of the head bolts 14, the diameter of the head bolt 14 and the diameter of the camshafts 2 or 3. When the distance between the two camshafts 2, 3 is reduced beyond the above mentioned sum, it becomes impossible to fasten with the head bolts 14 the cylinder head 1 to the cylinder block 13 after installation of the camshafts 2, 3 on the cylinder head 1.
For this reason, in the prior art arrangement, it has been practiced that the cylinder head 1 is first fastened to the cylinder block 13 with the head bolts 14 and thereafter the camshafts 2, 3 are disposed over the cylinder head 13 and supported by the bearings 4, 5, resulting in a poor assembling efficiency in the manufacture of the valve operating device and a poor working efficiency in the maintenance and care thereof.